Rock bolts for supporting structures e.g. roofs of passageways in mines are well known. There are many different types of rock bolts. A rock bolt generally consists of an elongate shank (length will generally depend upon the material which the rock bolt is intended to secure) having a distal end (the end which in use is fixed furthest within the rock), and a proximal end (the end, in use, which is closest to the surface of a rock and, in many cases, may actually project from the rock surface), or “tail end”.
Rock bolts are fixed in elongate boreholes (not much wider than the rock bolt) which is drilled in the rock. In use, a bearing plate is secured at the tail end of the rock bolt fast against the rock surface. The rock bolt and bearing plate assembly operate to support the rock. Many rock bolts may be used to support structures. For example, in mines rock bolts may be used to support passageways.
Installation usually requires drilling of the borehole by using a drill rig and a drill steel (a long steel rod with a drill bit on the end). The drill steel is then removed from the borehole. Resin (or “grout”) is inserted into the borehole, then the rock bolt itself is inserted and tightened up against the bearing plate.
Some rock bolts incorporate point anchoring mechanisms, which can be manipulated post insertion of the rock bolt to mechanically interfere with walls of the borehole in order to firmly secure the rock bolt.
The conventional procedure for installing rock bolts involves drilling a bore hole using a drill steel, removing the drill steel, inserting resin and a rock bolt and securing the rock bolt. “Self drilling” rock bolts are also known. These generally incorporate a drill bit as part of or connected to the distal end of the rock bolt, a tail end being attachable to a drill rig in order to drill the bore hole with the rock bolt. Once the hole is drilled, the rock bolt is retained in the hole.
One such self drilling rock bolt is disclosed in the Applicant's co-pending Australian patent application number 2006903922, entitled “Rock Bolt” and filed on 20 Jul. 2006. The disclosure of this provisional patent application is incorporated herein by reference. This earlier application discloses a self drilling rock bolt which includes a point anchoring mechanism.
Rock bolts are required to be high strength, typically over 30 tonnes ultimate tensile strength. Rock bolts are typically bonded to the borehole walls by resin. It is advantageous for the surface of the rock bolt to be deformed in order to provide high bond strength between the bolt/resin/rock interfaces.
Self drilling rock bolts have typically been rebars (strong steel bars) having an axially extending central path for water passage (and post grouting). The cost of making such hollow steel bars is quite high, and is uneconomical for high density rock support required by many underground mines.
It has been proposed to use a solid rebar with an outer sleeve for water passage during drilling. The outer sleeve, however, typically reduces the bonding between the rebar and the bore hole wall.
It has also been proposed to use high strength pipe which has limited surface deformations, if any, and is expensive and difficult to manufacture in the required high strength material.